With the increase in the density and speed of processors, the power consumption in the processors have been significantly increased in recent years, and along with this, various problems due to heat generation by the processors have arisen. Especially in a high-end server in which multiple processors are densely integrated in the same housing, how to allow heat produced to efficiently be escaped is becoming a critical factor that determines the design of the system. For example, a large heat sink is provided or multiple cooling fans are operated in a system to remove heat. However, heat increase that cannot be suppressed is becoming an impediment to improvements in performance. Because a high availability is required of high-end servers, the systems are not allowed to be shut down. Therefore, a heat removing mechanism in which a malfunctioned cooling fan can be replaced while the system keeps operation is provided in some system. In other systems, a mechanism that increases the rotation speed of the other cooling fans to compensate for a loss of the heat removing capacity is employed if some of the cooling fans malfunction. In yet other systems, a mechanism is provided that decreases the operating frequency of all processors by an equal degree to prevent overheating of the entire servers if a certain processor overheats. Redundant heat removing mechanisms as described above are used to improve the availability of systems, notably high-end servers for which heat removing is an important issue. As a result, the costs of the servers have been significantly increased. Further, these redundant heat removing mechanisms have their limits. It is often the case if a number of cooling fans fail at a time, the remaining cooling fans cannot sufficiently remove heat and the temperature of processors increases to a hazardous temperature in a few minutes, resulting in emergency shutdown of the system.
Because of such imperfection, service personnel are stationed in an environment such as a mission-critical system in a company that is not allowed to go down, so that failed parts of the system can be immediately replaced, which leads to a further cost increase. Furthermore, such mechanisms are intended to prevent only hardware damage or system shutdown. Accordingly, they are not designed to maintain a high service throughput in case of hardware overheating.
The following documents are considered:
                [Patent Document 1] Published Unexamined Patent Application No. 9-305268        [Patent Document 2] Published Unexamined Patent Application No. 11-191016        [Patent Document 3] Published Unexamined Patent Application No. 2002-202893        [Patent Document 4] Published Unexamined Patent Application No. 8-6681        
Various techniques that control power consumption with higher precision have been proposed. In particular, dedicated hardware circuitry is provided in a processor unit that reduces the operating frequency of the processors to control power so as to keep their temperatures within a predetermined range when the temperature of the unit increases (see Patent Document 1). Another technique has been proposed in which processing by a processor cooled by a cooling fan is stopped if a malfunction of the cooling fan is detected (see Patent Document 2). Yet another proposed technique controls task scheduling for allocating tasks to processors in an asymmetrical multiprocessor system so as to maintain the power consumption in the entire system below a predetermined value (see Patent Document 3). Another proposed technique reduces the operating frequency of a processor if the processor is found to be in an idle state in which it is not performing a task (see Patent Document 4).
The following problems are solved by the present invention. Firstly, according to Patent Document 1, circuitry dedicated to power control must be provided in processors. Therefore, it is difficult to implement the technique in computers such as low-cost servers and personal computers that are made up of off-the-shelf components. According to Patent Document 2, when a malfunction of a cooling fan is detected, the processor cooled by the malfunctioned cooling fan is stopped even if the malfunction is a slight reduction in the rotation speed of the cooling fan. Consequently, the maximum performance of the processor that could be achieved by cooling the processor with the fan cannot be provided at the reduced rotation speed and therefore the performance of the overall computer is excessively reduced. The system described in Patent Document 3 performs task scheduling in order to reduce power consumption. Therefore, the system can degrade the performance, rather than increasing processing speed or reducing waiting time, which are purposes of typical task scheduling. The technique described in Patent Document 4 cannot reduce the operating frequency of an overheated processor unless an idle state of the processor is detected.